This invention concerns pumps and more particularly impellers for non positive displacement pumps.
A particular type pump impeller which has long been known is comprised of a stack of spaced apart discs. This type of pump uses a boundary layer effect causing a boundary layer adhesion of the fluid to the disc faces to cause liquid to be pumped upon rotation of the impeller, creating pressure and flow at an outlet in an enclosing pump housing.
Another well known impeller configuration has a series of radially extending vanes, which accelerate the fluid by directly acting on the fluid with the faces of the vanes as the impeller rotates.
The spaced disc impellers have the advantage of largely eliminating the low pressure regions and cavitation characteristic of vane pumps. Disc impellers also are more efficient in pumping viscous fluids and can pump fluids having entrained abrasives with less impeller wear.
On the other hand, the induced flow rate per unit area of the discs is low, and reduced efficiency is encountered in this type of impeller at high flow rates.
Turbulence and cavitation can still occur as the fluid exits the outer perimeter of the disc and flows into the volute space in a confining housing. Further, there is a tendency for adhesion of the fluid to the outer perimeter of the discs to create turbulence and thereby reduce the efficiency of the pump.
It has been heretofore been proposed to combine a vaned and spaced disc impeller, see U.S. Pat. No. 4,255,081 issued on Mar. 10, 1981 for a "Centrifugal Pump".
This combination is intended to provide most of the advantages of both types of impellers.
However, cavitation may still occur of the outer ends of the vanes and turbulence in the regions beyond the impeller perimeter.
It is the object of the present invention to provide an improved pump impeller of a hybrid vane-disc type in which the tendency for cavitation is greatly reduced and which operates more efficiently than prior designs.